Method of preparing lithographic printing plates



'\image areas transfers thereto.

United States Patent f 3,106,158 METHOD OF PREPARING LITHOGRAPHICPRINTING PLATES viichael Michalchik, Levittown, N.J., assignor to RadioCorporation of America, a corporation of Delaware No Drawing. Filed Jan.22, 1962, Ser. No. 167,908

5 Claims. (Cl. 101-1492) This invention relates to improved lithographicprint ing plates and, more particularly, to the use of improvedconversion solutions for preparing lithographic printing plates fromprints made by an electrostatic duplicating process.

Lithographic printing is a Well known form of planographic printing. Ingeneral, the process includes printing from a flat (or planographic)plate and it depends upon different properties of the image andnon-image areas for printability. In the ordinary lithographic printingprocess a printing plate is prepared by afiixing to a Water-attractive,hydrophilic surface, a water-repellent hydrophobic image, usuallygreasy, resinous or waxy in nature. An aqueous wet-out liquid is thenapplied to the surface so imaged. This liquid wets all portions of thesurface not occupied by the hydrophobic image matetrial but does not wetthe hydrophobic image.

An inking roll coated with a grease-base lithographic printing ink ispassed across the surface of the printing plate, leaving a film of inkupon the ink-receptive hydrophobic irnage areas. No ink is left on thenon-image areas which are covered by a film of ink-repelling wetoutliquid.

When the inked plate is brought into contact with another ink-receptivesurface, some of the ink from the be transferred directly to a papersheet but generally is transferred, first, to a rubber offset blanketwhich, in .turn, is used to transfer the print to a final paper sheet.For each print made during a printing run, the printing plate isdampened with the wet-on or, as it is usually called, the fountainsolution, for the purpose of keeping the non-image areas wet. I

In the past, plano graphic printing plates have been made of zinc oraluminum sheets having surfaces especially treated to render themhydrophilic. For shorter printing ,runs, paper base plates coated with apigment dispersed in a hydrophilic adhesive, have been used.

Images have generally been placed on the metal plates photographicallywith the aid of a special photographically sensitized master, or,non-photographically, as with a grease pencil. Images have generallybeen placed on the paper base plates by typing using a special ribbon.

More recently, electrostatic methods have been used for applying imagesto lithographic printing plates. This type of process, in general,comprises placing an over-all, uniform, electrostatic charge on a platehaving a surface made of a photoconductive material, such as selenium,exposing the plate to an image of light and shadow to be reproduced,thereby removing the electrostatic charge in those areas of the platereceiving light, and thus producing an electrostatic charge pattern onthe plate surface exactly corresponding to the dark areas of theoriginal to be reproduced. The charge pattern is made visible bycontacting the surface heating the electrostatic charge pattern The inkimage may thus zinc oxide, and a resin binder.

3,106,158 Patented Oct. 8, 1963 IQC 2 with a finely dividedelectroscopic developer material (also referred to herein as a toner)bearing a charge opposite to that of the electrostatic image, wherebythe developer material deposits on the plate in image configuration.

The powder image must then be transferred to the printing plate surfaceand stabilized as by heating to a temperature sufiiciently high to fusethe developer material and cause it to adhere to the plate surface.

Still more recently, the above-described electrostatic method has beenimproved by eliminating the transfer step necessitated by using aselenium plate to first record the light image. In the improved methodthe image is recorded and fixed directly upon an improved type ofprinting plate. The improved type of printing plate has a printingsurface composed of a powdered photoconductive material, preferably aparticular grade of photoconducting The resin binder must have goodelectrical insulating properties since the plate must be able to hold anelectrostatic charge for an appreciable length of time in darkness. Ithas further been found that resins which are hydrophilic are unsuitablesince they apparently attract sufficient Water to lower the surfaceresistance to an unsuitable level. This permits too much lateral leakageof the electrostatic-charge and prevents storage of an electrostaticcharge pattern long enough to permit satisfactory developing. It hastherefore been found necessary to limit the resin binders in the abovedescribed recording composition to those which are hydrophobic.

As previously pointed out, a lithographic printing plate surface musthave hydrophilic properties in its non-printing areas. Thus, in order touse a conventional zinc oxide-resin coated electrostatic recordingsurface as a lithographic printing plate, the hydrophobic surface mustfirst'ebe converted to hydrophilic. Solutions for accomplishing thisconversion have been devised and have been used with more or lesssuccess. Some of these, however, require an unduly long treating timeand do not work satisfactorily unless the plate is completely immersedin the solntionfor an appreciable period of time. It is desirable to beable to accomplish the conversion step in a highly effective manner butin as short a time as possible and with assimple a treatingtechnique aspossible.

In the case of zinc oxide-resincoated plates, the conversion solutionreacts with the -zinc oxide to produce an insoluble compound thatattracts water to its surface. A

further requirement for a satisfactorily acting conversion solution isthat it shall not undercut the fused toner deposit on the printingsurface or weaken the deposit in any other manner.

One object of the present invention is to provide an improved conversionsolution for converting a zinc oxidehydrophobic resin surface fromhydrophobic to hydrophilic Example I A planographic plate is prepared byspray-coating an aluminum sheet to a thickness of about 1.0 (afterdrying) with the following composition:

Pliolite S-7 (butadiene-styrene copolymer of the Goodyear Tire andRubber Company -gm 270 SR-82 (silicone resin of the General ElectricCompany) gm 135 Zinc oxide (Green Seal #8, New Jersey Zinc Company) gm800 Xylene ml 2,000 Dye (2% solution of sodium fluorescein in methanol)ml 11 The plate is permitted to dry in air until the xylene hasevaporated.

A printing image is applied to the plate as follows:

First, the surface of the plate is given a uniform electrostatic charge(in the dark) by passing a high voltage corona discharge unit across it.Next, the charged suriiace is exposed to a positive image pattern of thesubject matter is to be printed so that a charge image of the subjectmatter remains on the surface of the printing plate.

Then the charge image (again in the dark) is developed by sweepingacross the surface a mixture of iron particles free from grease andother contaminants and having a greatest dimension between about 0.002"and 0.008", and an easily fusible powder or toner above iron in thetriboelectric series. The iron merely serves as a carrier for the tonerparticles. Particles of toner are attracted to the electrostatic chargeimage and deposit in the charged areas.

A suitable toner can be made by, first, melting then cooling and ballmilling together 200 g. of Piccolastic resin 4358A (PennsylvaniaIndustrial Chemical Co.), 12 g. carbon black G, 12 g. Nigrosine SSB dyeand 8 g. Isol Black dye. The ground powder is finally screened through a200 mesh screen.

The toner particles are fused to the zinc oxide-resin coated surface byheating with an infra-red lamp for about 20 seconds at 190 C. Thiscompletes the formation of the printing image.

Using the printing plate as described'above, a planegraphic printing runis carried out as follows.

A conversion solution in accordance with the present invention is madeup with the composition:

Potassium cobalt'icyanide g 5 Glacial acetic acid ml 2 Ammoniumdichromate g 0.10 Water ml 100 The solution is swabbed on theimage-containing surface of the plate so that the entire surface isthoroughly and uniformly wetted. The solution is allowed to react withthe zinc oxide-resin surface for 20-30 seconds. The cobalticyanidereacts with at least part of the zinc oxide in the non-image areasforming an insoluble zinc cobalticyanide which is hydrophilic.

V In order to increase contrast in the printed product, the plate isnext treated with a solution of Platex (Addres-sograph-Multigraph Corp,Cleveland, Ohio). P-la'tex is believed to comprise a nickel salt,ammonium acid phosphate, butanal, cellulose gum, diethylene glycol,formaldehyde, and water. The solution is applied in the form of onevolume Platex to 3 volumes water. This solution reduces any tendency ofthe conversion solution to wet the black areas of the printing surface.

The next step in the process is to swab the printing surface of theplate with the fountain solution to be used on the press during theprinting run. Sufiicient fountain solution is applied in this manner towash off the Platex.

The plate is mounted on a lithographic press and wet with a fountainsolution each time a print is made. A suitable fountain solutioncomprises:

Sodium ferrocyanide 0.1% byweight. Glacial acetic acid 0.5% by volume.Glycerine 1.0% by weight. Water balance.

' Example 11 As in Example 1, a printing plate was made up by coating analuminum sheet with the following composition:

Parts by Weight 'Piccolastic resin D 60 Piccolastic resin C-l25 40Carbon black 7 Calco Black Dye (F-4610B, Allied Chemical and Dye Corp.)2

The resin is melted and the carbon black added to the melt, followed bythe dye. The ingredients are thoroughly mixed at a temperature of about175 -200 C. The mixture isthen cooled and the hard cake is crushed andscreened to 20 mesh. This powder is micronized to an average particlesize of 5-10 microns.

After this toner is applied to the charge image, it can be fused to thezinc oxide-resin surface by heating to the relatively low temperature ofC. lfOl' 30 seconds.

The remainder of the process is the same as in Example I except that theamount of potassium cob-alticyanide in the conversion solution is 2.5 g.

Example III A printing plate was made up by coating one side of a sheetof paper with the following composition:

Silicone resin solution (SR-'82, 60% solids content The silicone resinsolution and toluene are mixed, then the zinc oxide is added withstirring. This mixture is dispersed in a blender until a smoothpaint-like mixture is obtained. The mixture is applied uniformly to oneside of the paper sheet with a doctor blade. sensitizing dyes such asRose Bengal 01' Sodium Fluorescein may be included in the composition.As in the previous examples, the coating must be dried by driving offthe solvents.

This plate is used for lithographic printing as in the previousexamples.

The improved conversion solution of the present invention may be variedsomewhat within the scope of the invention. Other alkalicobalticyanides, for example, sodium or lithium, may be used instead ofthe potassium salt given in the example. The concentration of this saltmay be varied between about 0.5 and 5.0 g. per 100 ml. of Water. But thesolution may be much more dilute if the plate is dipped instead ofswabbed. The acetic acid may be varied between about one and four ml.per 10 0 ml. of water and any one of a number of equivalent acids,previously mentioned, may be used. Also, the dichromate may be anyalkali metal dichromate instead of ammonium dichromate.

The hydrophobic resin used as the binder with the Zinc oxide of theprinting plate is not critical. However, the a resin binder must not behardened to the extent that the conversion solution cannot penetrate toact on the zinc oxide. What is claimed is: V 1. A planographic printingplate having non-image areas comprising zinc oxide and a hydrophobicresin binder, said areas having been rendered hydrophilic by convertingat least part of said zinc oxide to Zinc cobalticyanide.

2. A planographic printing plate having non-image areas comprising zincoxide and a hydrophobic resin binder, said areas having been renderedhydrophilic by treating with an aqueous solution comprising an alkalimetal cobalticyanide, an acid and an alkali dichromate to convert atleast part of said zinc oxide to zinc cobalticyanide.

3. A planographic printing plate having non-image areas comprising zincoxide and a hydrophobic resin binder, said areas having been renderedhydrophilic by treating with an aqueous solution comprising potassiumcobalticyanide, acetic acid and ammonium dichromate.

4. In the method of phanographic printing wherein the printing plate hasnon-image areas comprising zinc oxide and a hydrophobic resin binder,the step of rendering said non-image areas hydrophilic which comprisestreating said areas with an aqueous solution comprising an alkali metalcobalticyanide, an acid and an alkali dichromate.

5. In the method of planographic printing wherein the printing plate hasnon-image areas comprising zinc oxide and a hydrophobic resin binder,the step of rendering said non-image areas hydrophilic which comprisestreating said areas with an aqueous solution comprising an alkali metalcobalticyanide, acetic acid and ammonium dichromate.

References Cited in the file of this patent UNITED STATES PATENTS2,957,765 Resetich Oct. 25, 1960 2,993,787 Sugarm-an July 25, 19613,001,872 Kurz Sept. 26, 1961

1. A PLANOGRAPHIC PRINTING PLATE HAVING NON-IMAGE AREAS COMPRISING ZINCOXIDE AND A HYDROPHOBIC RESIN BINDER, SAID AREAS HAVING BEEN RENDEREDHYDROPHILIC BY CONVERTING AT LEAST PART OF SAID ZINC OXIDE TO ZINCCOBALTICYANIDE.